The present invention relates to methods for determining acoustic parameters of liquids in an acoustic resonator, particularly a method for determining absolute acoustic values of the liquids from a sequence of acoustic liquid resonances, and a resonator arrangement for performing a method of this type.
The characterization of liquids on the basis of their acousto-mechanical properties using an acoustic resonator is generally known. An acoustic resonator is a container, whose shape is dependent on its application, in which a quantity of liquid is excited to mechanical oscillations which are reflected as sound waves between two diametrically opposite walls of the container. The excitation is performed using electrical or magnetic elements (electroacoustic or magnetoacoustic transmitters), which are acoustically coupled to the container. The liquid oscillations have resonances at certain frequencies, the reflected sound waves interfering with one another in such a way that they amplify to a spatial standing wave. The resonance frequencies depend on the shape and the composition (e.g., material, surface properties) of the container and of the properties of the liquid to be investigated in the resonator. The resonance frequencies are determined, for example, by detecting electrical measured variables (e.g., current, voltage) at the transmitter or by detecting the oscillation amplitudes using acoustically coupled receiver elements.
Numerous techniques are known for oscillation excitation, oscillation measurement, and analysis of the frequency characteristic of the liquid. For example, in WO 94/24526 an ultrasonic measurement device having a non-piezoelectric resonator chamber body and electroacoustic transducers positioned on its outside is described. The inner walls of the resonator chamber body form a resonator being as ideal as possible for generating standing linear sound waves in the liquid. For this purpose, the resonator chamber body must be produced with special precision. In addition, the most effective possible coupling of the external electroacoustic transducers to acoustically conductive layers is provided. The area of application of the conventional ultrasonic measurement device is restricted because of the complex construction of the resonator chamber body.
A method for evaluating acoustical-electrical measured variables to obtain acoustic parameters or variables derived therefrom is known from WO 95/12123. This method requires performing comparative measurements using a measurement resonator, which is filled with a liquid sample, and a reference resonator, which contains a known liquid. The performance of reference measurements is disadvantageous since they represent an additional measurement outlay and possibly impair the precision of the measured variable analysis.
A measurement method using acoustic standing cylinder waves is known from U.S. Pat. No. 5,533,402. This technology also has the disadvantage that reference measurements must be performed to obtain acoustic parameters. A further disadvantage relates to the cylinder resonators described in U.S. Pat. No. 5,533,402, which must again be produced with high precision to generate the cylindrical waves.
Cylinder resonators having curved electroacoustic transducers are known from the book “Ultrasonic Interferometers” by V. Ilgunas et al., (Russian language, Verlag Mokslas, Vilnius 1983). Using these resonators, standing wave fields may be generated whose resonances have a constant frequency interval. The frequency interval is nearly a linear function of the mutual spacing of the curved transducers. To avoid cylindrical resonances, the resonators are constructed as open resonators. This is disadvantageous since the transducers must be dipped into the sample liquid. A closed sample chamber is not produced.
Physical formulas are also known for theoretically calculating liquid resonances in ideal resonators. In these formulas, which are described below, some real conditions of the acoustic excitation, particularly the inclusion of the natural resonance of the transducer used for the excitation on the observed resonance frequencies of the liquid, are not taken into consideration (see J. P. M. Trusler in “Physical Acoustics and Metrology of Fluids”, Adam Hilger Verlag, Bristol, Philadelphia, New York, 1991, pp. 52–89).
The object of the invention is to provide improved methods and devices for determining acoustic parameters and/or values of liquids derived therefrom, using which the disadvantages of the conventional techniques are avoided. In particular, acoustic characterization of the liquids with increased precision is to be made possible, without a reference measurement being necessary. The method is also to be implementable using a simplified resonator construction.